'\" t
.\" Copyright 2022 Alejandro Colomar <alx@kernel.org>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH strncat 3 2023-12-05 "Linux man-pages 6.7"
.SH NAME
strncat
\-
append non-null bytes from a source array to a string,
and null-terminate the result
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <string.h>
.P
.BI "char *strncat(char *restrict " dst ", const char " src "[restrict ." ssize ],
.BI "              size_t " ssize );
.fi
.SH DESCRIPTION
This function appends at most
.I ssize
non-null bytes from the array pointed to by
.IR src ,
followed by a null character,
to the end of the string pointed to by
.IR dst .
.I dst
must point to a string contained in a buffer that is large enough,
that is, the buffer size must be at least
.IR "strlen(dst) + strnlen(src, ssize) + 1" .
.P
An implementation of this function might be:
.P
.in +4n
.EX
char *
strncat(char *restrict dst, const char *restrict src, size_t ssize)
{
    #define strnul(s)  (s + strlen(s))
\&
    stpcpy(mempcpy(strnul(dst), src, strnlen(src, ssize)), "");
    return dst;
}
.EE
.in
.SH RETURN VALUE
.BR strncat ()
returns
.IR dst .
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR strncat ()
T}	Thread safety	MT-Safe
.TE
.SH STANDARDS
C11, POSIX.1-2008.
.SH HISTORY
POSIX.1-2001, C89, SVr4, 4.3BSD.
.SH CAVEATS
The name of this function is confusing;
it has no relation to
.BR strncpy (3).
.P
If the destination buffer does not already contain a string,
or is not large enough,
the behavior is undefined.
See
.B _FORTIFY_SOURCE
in
.BR feature_test_macros (7).
.SH BUGS
This function can be very inefficient.
Read about
.UR https://www.joelonsoftware.com/\:2001/12/11/\:back\-to\-basics/
Shlemiel the painter
.UE .
.SH EXAMPLES
.\" SRC BEGIN (strncat.c)
.EX
#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
\&
#define nitems(arr)  (sizeof((arr)) / sizeof((arr)[0]))
\&
int
main(void)
{
    size_t  n;
\&
    // Null-padded fixed-size character sequences
    char    pre[4] = "pre.";
    char    new_post[50] = ".foo.bar";
\&
    // Strings
    char    post[] = ".post";
    char    src[] = "some_long_body.post";
    char    *dest;
\&
    n = nitems(pre) + strlen(src) \- strlen(post) + nitems(new_post) + 1;
    dest = malloc(sizeof(*dest) * n);
    if (dest == NULL)
        err(EXIT_FAILURE, "malloc()");
\&
    dest[0] = \[aq]\e0\[aq];  // There's no 'cpy' function to this 'cat'.
    strncat(dest, pre, nitems(pre));
    strncat(dest, src, strlen(src) \- strlen(post));
    strncat(dest, new_post, nitems(new_post));
\&
    puts(dest);  // "pre.some_long_body.foo.bar"
    free(dest);
    exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.in
.SH SEE ALSO
.BR string (3),
.BR string_copying (7)